The present invention generally relates to gemstone observation, and more particularly to an apparatus and method for isolating and observing the fire of a gemstone.
The quality and value of faceted gemstones are often described in terms of the “four C's”: carat weight, color, clarity and cut. Carat weight is the most objective, because it is measured directly on a balance. Color and clarity are factors for which grading standards have been established by the Gemological Institute of America (GIA), among others.
Cut is much less tractable. Unlike color and clarity, for which a legacy of teaching, trading, and laboratory practice have created a general consensus, there are a number of different grading systems for grading cut of a gemstone. Inherent in most of these systems is the premise that there is one set, or a narrow range, of preferred proportions for some gemstones, and that any deviation from this set of proportions diminishes the appearance or attractiveness of the gemstone. However, under this premise, gemstone cutters typically apply these proportions only to obtain the largest possible size gemstone from an uncut stone, without specific regard to the stone's eventual appearance.
Most gemstones are a convex polyhedron which can be specified according to a number of parameters. FIG. 1 illustrates various parameters that define the proportions of one type of gemstone, a round brilliant cut (RBC) diamond. This type of gemstone can be specified according to eight parameters. Crown angle is the angle, in degrees, between the bezel facets and the girdle plane. Pavilion angle is the angle, in degrees, between the pavilion mains and the girdle plane. Table size represents the width of the table as a percent of the girdle diameter. Culet size represents the width of the culet as a percent of the girdle diameter. Star length is a ratio of the length of the star facets to the distance between the table edge and girdle edge. Lower girdle length represent a ratio of the length of the lower girdle facets to the distance between the center of the culet and the girdle edge. Girdle thickness is preferably measured between bezel and pavilion main facets, and is expressed as a percentage of girdle diameter. Finally, girdle facet number is the total number of facets on the girdle. Given a number of gemstones of the same color, weight and clarity, varying any of the above parameters produce different appearances.
Other than color, weight, and clarity, gemstone appearance has historically been described chiefly in terms of three aspects: brilliance, scintillation, and fire. While interrelated, these aspects can be characterized independently. Brilliance, or brightness, generally refers to the level of white light returned through the crown of a gemstone to an observer overhead. Scintillation refers to flashes of light reflected from the crown of a gemstone, particularly as the gemstone is rotated or tilted. Fire is the result of the light-dispersive quality of a gemstone, and refers to visible rays or flares of colored light returned by the gemstone.
It is believed that with knowledge about how cut relates to each of these aspects, alone or in combination, then perhaps improved cut parameters can be established to yield more attractive, and thus more valuable, gemstones. Unfortunately, each aspect above represents a complex concept without a precise mathematical definition, making it very difficult to measure on actual gemstones.
Models have been developed for some aspects, however. For example, GIA developed a mathematical model for brilliance, discussed in Modeling the Appearance of the Round Brilliant Cut Diamond: An Analysis of Brilliance, by Hemphill et al., Gems & Gemology, Vol. 34, No. 3, pp. 158-183, the contents of which are incorporated by reference herein in their entirety and for all purposes. GIA's brilliance model uses a simulated round brilliant cut (RBC) diamond and a modeled light source of diffused, hemispherical white light shining on the crown. Then, researchers used computer simulation techniques to examine mathematically how millions of rays of light from the virtual light source interact with the virtual gemstone. This model generated images and a numerical measurement of the optical efficiencies of the gemstone called weighted light return (WLR). The WLR is a weighted sum of the amount of light returned through the crown of the virtual diamond to all positions of observation above the girdle. Thus, WLR approximates overall brilliance in an environment with even diffused lighting and no objects, such as an observer, in the environment.
Similar assumptions and qualifications were used in developing a metric for fire. See Modeling the Appearance of the Round Brilliant Cut Diamond: An Analysis of Fire, and More About Brilliance, Gems & Gemology, Vol. 37, No. 3, pp. 174-197, the contents of which are also incorporated by reference herein for all purposes. While brilliance is emphasized with diffuse illumination found in most common lighting environments, fire is best observed using a highly directed, narrow beam of light, referred to herein as “spot lighting.” Accordingly, GIA chose to model the directed lighting as a bright point source of illumination located very far from the gemstone, i.e. at infinite distance, centered over and directed toward the gemstone's table. Under these conditions, the unpolarized light rays entering the crown facets are parallel to one another and perpendicular to the table, to illuminate the entire crown. The metric derived—dispersed colored light return, or DCLR—describes the potential of an RBC gemstone with certain proportions to display dispersed colored light when viewed face-up.
Fire is the most difficult aspect of a gemstone to observe. Fire is often mixed with scintillation, the white light flashes that obscure the rays of colored light. Further, white light in general, either from the lighting environment itself or returned from the gemstone as brilliance, can overwhelm and suppress the visible effects of fire.
A particular type of directed light source, for example one which approximates the GIA modeled light source, can isolate or enhance observable fire. However, several problems exist with finding and using such a source. Commercially-available narrow beam spot lights are not sufficiently directed, and allow too much white light from too many angles to reach a gemstone being observed, obscuring the fire. On the other hand, some highly directed light sources, such as lasers or light emitting diodes, radiate at too little of the visible spectrum for viewing the full range fire-based color separation. What is needed is a apparatus and method by which a white light source is channeled directly to a gemstone, in order to better isolate, observe and measure fire.